IME, length of the bridle path depends somewhat on the discipline the horse is being trimmed for. Western disciplines typically call for (and 4-H generally teaches, regardless of discipline in my area) the ear-length rule. Hunters and dressage horses more often have shorter bridle paths, not much wider than the crownpiece of the halter or bridle. I imagine that eventing follows a similar guideline. And, of course, there are Arabs, Morgans, and gaited breeds that have much longer bridle paths intended to highlight the conformation of the neck and throatlatch.
I love this book: http://www.amazon.com/Grooming-Win-Groom-Braid-Prepare/dp/0470047453/ref=sr_1_1?ie=UTF8&qid=1307880634&sr=8-1
Some of the info is a little bit dated in terms of what is "in" in the show ring today, but much of it is timless, tried and true guidelines to prepping a horse to make a great impression in the show ring.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown